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import { TreeNode } from "../mod.ts";
export default function flatten(root: TreeNode | null): void {    const list: TreeNode[] = [];    preorderTraversal(root, list);    const size = list.length;    for (let i = 1; i < size; i++) {        const prev = list[i - 1];        const curr = list[i];        prev.left = null;        prev.right = curr;    }}function preorderTraversal(root: TreeNode | null, list: TreeNode[]) {    if (!root) return;    list.push(root);    preorderTraversal(root.left, list);    preorderTraversal(root.right, list);}